In the present practice of producing a steel strip or sheet, the starting material is a steel slab which is produced by the ingot process including blooming or by a casting process such as a continuous casting process. The thus-obtained steel slab is cooled down to an ambient temperature. Thereafter, this slab is heated up to a temperature in the range of 1200-1300.degree. C. for more than three hours in a slab reheating furnace. It is then fed to a hot rolling mill and hot rolled into the desired thickness.
From the time of the development of the continuous casting technique, it has been the desire of those in the art to be able to roll continuously a cast steel slab having a high temperature directly, i.e. without the necessity of reheating it. This process of directly rolling the cast steel, hereinafter called a direct hot rolling process, is well-known and well-established and various ways of carrying out this process have been proposed. The main object of this direct hot rolling process in the past has been to make the processing steps of casting and hot rolling continuous and to save energy as compared with the conventional process in which the slab is cooled down to an ambient temperature and reheated in a slab-reheating furnace prior to hot rolling. There has been no consideration of the technical problems to be solved in this direct hot rolling process and how this process would influence the quality of the final product from a metallurgical viewpoint.
The present inventors have carefully studied the relationship between a heat diagram for a steel slab and that for a hot rolled steel strip and have discovered an important relationship between the two steps.